1. Field of the Invention
For both environmental and industrial hygiene reasons, there is increasing concern about emissions from engines and other industrial apparatus, and the regulations concerning such emissions are becoming increasingly strict. Realistically, it is necessary to attempt to design or modify such apparatus so that, in normal operation, it will emit no hazardous fluids. Where the fluid is a gas, there is particular concern, since, once emitted, the gas is much more difficult--often impossible--to contain and recover.
Engines that use natural gas as fuel are often used where natural gas is readily available. For example, they are often used to run compressors associated with natural gas pipelines. It is much more efficient to use the natural gas at hand as fuel, rather than bringing in some other form of fuel. Many such engines were put into service when natural gas emissions were of little or no concern. Now, however, natural gas emissions are of concern, particularly in environmentally sensitive areas such as California. It is desirable to find a simple way to make engines already in the field comply with stricter regulations, so that they need not be replaced. To the extent that new engines are put into service, it is desirable to minimize the amount of redesign and retooling necessary to meet current emissions standards.
Among the main potential leak points in such an engine are the fuel injection valves. If emissions from such valves can be controlled, little or no engine redesign or modification may be necessary. It is, in turn, desirable to modify existing valve technology as simply and inexpensively as possible.
2. Description of the Prior Art
Much different types of valves, e.g. butterfly valves that open and close conduits, have been designed so that an inert buffer gas can be injected into a potential leak space at a pressure greater than that being sealed against. By virtue of this higher pressure, the buffer gas may leak into the vessel or process, rather than leakage occurring in the opposite direction. Any gas that leaks to the atmosphere will be buffer gas, which is chosen so as to be harmless. Examples are found in U.S. Pat. No. 3,979,104 to LaCoste et al. and WIPO Publication No. W092/16776 to Calvin.
The structures of these valves are very different from that of a fuel injection valve. Furthermore, the seal structures do not experience the same kind of service. A butterfly valve for a conduit is opened or closed relatively infrequently. In an engine, there is a reciprocal stroke of the valve stem of each fuel injection valve for every firing of the corresponding cylinder. Thus, these stems are constantly and rapidly reciprocating in use, and sealing is required about the circumference of such stems. Mechanical seals can develop small leaks very rapidly in such service.
It has also been known to use buffer fluids in the seal areas about piston rods, as discussed in "Automated Compressor-packing System Meets California Emissions Reg," Oil and Gas Journal, Jul. 6, 1992, PennWell Publishing Company, and in "Pistons, Rods, Rings Respond to Loading," Plant Services, July 1992. Although piston rods do reciprocate rapidly, the structural differences between this general type of apparatus and that of a fuel injection valve are, perhaps, even greater than that between butterfly valves and fuel injection valves.
That is particularly true of a particular type of fuel injection valve, which has even greater structural dissimilarity from the above art, but which has become increasingly popular for natural gas engines. Specifically, this is a type that includes an outer housing or "cage" that at least partially receives and surrounds an inner housing. It is the inner housing that carries the valve seat and in which the valve stem and element work. Valves of this type have been sold for many years under the trademark "Rebecca." An example is illustrated in U.S. Pat. No. 4,365,756 to Fisher. In other versions, chevron seals have been used.
While offering many advantages over older single housing designs, these dual housing valves present even greater challenges in terms of sealing. In addition to the locus between the valve stem and the housing ID, there is now a second annular locus, between the two housings, that must also be sealed. In many of the dual housing valves, it would be a particular difficulty to use a buffer gas in both of these loci. If it could be done at all, it might require separate gas injectors for the two loci, respectively, and/or the need for a complicated system of flow passages for the buffer gas.
U.S. Pat. No. 3,319,647 to Morain discloses an exhaust valve for an expansion type engine that handles extremely cold (cryogenic) fluids. This design does illustrate a system of passages communicating with two annular loci. However, these passages are not designed to inject a buffer gas (or anything else), but rather to allow cold fluid that does leak to enter a stagnation zone. The reason for this is understood to be that any fluid that contacts the outermost seals will not be so cold, and will therefore be less likely to interfere with the mechanical seals' effectiveness.
In other words, Morain seems to simply accept the idea that there is no way to prevent some leakage outwardly from the engine, and contents himself with trying to attenuate the ill effects of cold temperatures on the mechanical seal. Not only would his approach not work in the context of more typical engines, but even his structure would be unsuitable for adaptation to that service. By way of example, many of his seals are not fluid pressure responsive. The only seal arguably provided axially inwardly of the radially outermost annular locus is simply the face-to-face contact between two axially abutting parts 10 and 36 of the housing in question, and this would more likely leak to atmosphere to an unacceptable degree if fluid were injected at a pressure even higher than that in the engine.
The upshot of all this is that, to the present inventor's knowledge, no prior fuel injection valves, especially for natural gas engines, have been provided with buffer gas systems. The rather primitive "state of the art" for dealing with fugitive emissions in this art is for a person to frequently pass a detector known as a "sniffer" about the likely leak points of an engine. If a leak is thus detected, the system is depressurized and shut down, and the seals are replaced. Not only is the monitoring expensive, time-consuming and bothersome, but given current seal systems, the seals of many valves do develop leaks and require replacement relatively often, resulting in further expense, bother, and downtime for the engine.